1. Field of the Invention
This invention relates to power factor correction and more particularly to circuit for providing such correction in a power supply.
2. Description of the Prior Art
Switch mode power supplies of the off-line AC to DC converter type are used in a wide variety of applications. Such supplies may be used in the computer industry to run computers and peripherals or in the telecommunications industry as a battery charger or eliminator. No matter where such supplies are being used, there is a growing awareness of the power factor correction problem.
In a typical power supply of the off-line AC to DC converter type, the input current to the supply is essentially in phase with the input voltage to the supply. The input current does, however, have a very high harmonic content. Power factor is defined as the ratio of the input watts to the product of the RMS values of the input voltage and input current. The ratio is affected by the phase relationship of the fundamental of the input voltage and input current and the harmonic content of the input current. As a result of the very high harmonic content in the input current, the power factor for the typical converter type power supply may be in the order of 0.62. Ideally, that power factor should be unity. It is therefore desirable to include in the power supply circuitry which attempts to correct the power factor and make it as close to unity as is possible.
There have been numerous attempts in the art to design power factor correction circuits. Several such correction circuits and the power supplies in which they are used are described in U.S. Pat. Nos. 3,365,657; 4,193,111; 4,437,146; 4,384,321 and 4,412,277.
In the U.S. Pat. No. 3,365,657, the power factor correction circuit described therein is used to correct the power factor of a highly reactive variable load as seen by the power amplifying elements of the power supply. Such correction is accomplished by including in the output transformers, control windings which have a direct current applied thereto. That current is a function of the load circuit and varies the self inductance of the output transformers in such a manner as to provide a unity power factor as viewed from the transistors. The circuit described in the U.S. Pat. No. 4,193,111 accomplishes power factor correction by selectively operating switches to effectively connect an inductor to the positive or negative terminal of a storage capacitor or to zero volts, depending on the relationship of the current to a reference currents and whether the voltage across the AC ports is positive or negative. The U.S. Pat. Nos. 4,437,146, 4,384,321 and 4,412,277 describe other equally as complicated schemes for correcting power factor.
A further discussion of power factor and correction circuits therefor may be obtained by referring to the following publications:
(1) "Power Factor Correction--I"; "Power Factor Correction--II"; and "Power Factor Correction--III" Steve Smith, PCIM Magazine January 1987, February, 1987 and March 1987 respectively.
(2) "Input--Current Shaped Ac-to-Dc Converters", Final Report, California Institute of Technology, Power Electronics Group, May 1986, available from the National Technical Information Service as Document number N86-25693.
Referring now to FIG. 1, there is shown a simplified block diagram of the well known AC to DC converter type switch mode power supply 10. Supply 10 includes a rectifier 12 which converts the AC input voltage into a DC voltage. Connected to rectifier 12 are input filter capacitors 14. The capacitors 14, also known as energy storage capacitors, smooth the DC voltage. The voltage appearing thereacross is sometimes referred to as the bank voltage. Power supply 10 also includes power switches 16 which operate in the switch mode and provide the power conversion. The operation of the switches is controlled by control 18. The output voltage and current resulting from the operation of the switches is connected to the output side of power supply 10 by power transformer 20. Output filter 22 is used to filter the voltage and current appearing at the output of the supply. Control circuit 18 also includes a driving circuit (not shown) which is used to provide the actual signals to switch power switches 16.
Referring to FIG. 1A there is shown waveforms for the input current and input voltage to power supply 10. The waveforms shown therein are for supply 10 operating at essentially full load. As can be seen while the input current and input voltage are essentially in phase with one another the input current has a high harmonic content. As a result, the power factor of supply 10 is in the order of 0.62. The efficiency of supply 10 is in the order of 89.3%.
Power supply 10 of FIG. 1 does not include a power factor correction circuit. It is however, desirable that supply 10 include such a circuit. In addition, it is also desirable that such a circuit be relatively easy to implement and also be easily retrofittable into previously designed power supplies. It is also desirable that the basic architecture of the power factor correction circuit be expandable so that power factor correction can be provided over part or all of the operating range of supply 10. It is additionally desirable that the power factor correction circuit have minimal effect on the efficiency of supply 10, does not introduce any new radio frequency (r.f.) signals in the supply and also have the capability to suppress r.f. signals presently in the supply. The power factor correction circuit of my invention has these characteristics.